The invention relates to a multiple-ratio transmission mechanism in a powertrain for an automotive vehicle and to a control strategy for achieving smooth engagement and release of friction torque establishing elements during a transmission upshift event.
In a geared automatic transmission in an automotive vehicle powertrain having an engine or other torque source, a ratio change may be made from a so-called low ratio to a so-called higher ratio when a friction torque establishing element, such as a clutch or brake, is engaged in synchronism with disengagement of a companion friction torque establishing element. This is referred to as a ratio upshift. The friction torque establishing elements involved in the upshift may be referred to as an oncoming clutch or brake and an off-going clutch or brake. The upshift event is characterized by a preparatory phase, a torque phase and an inertia phase as the vehicle accelerates from a standing start.
In a conventional automatic transmission in a vehicle powertrain, the oncoming clutch torque capacity is controlled to increase from a low value during the torque phase. Simultaneous engagement of one clutch or brake and release of another results in a momentary activation of two torque flow paths through the gearing, causing a gear tie-up in which transmission output shaft torque decreases momentarily. This condition may be referred to as a “torque hole”. It occurs before the off-going clutch totally disengages.
Friction elements, such as disc clutches, band brakes and disc brakes, typically are actuated hydraulically under the control of a transmission control module, which disengages an off-going friction clutch or brake while simultaneously engaging an oncoming friction clutch or brake during an upshift in order to lower speed ratio. For purposes of the present description of the invention, the clutch and the brake will be referred to as friction elements.
During the preparatory phase, an automatic transmission control reduces off-going friction element torque capacity to prepare it for release as an actuator for the oncoming friction element is adjusted to prepare for its engagement. During the torque phase, the controller increases oncoming friction element torque capacity, which causes torque transmitted through the off-going friction element to drop quickly due to the transient gear tie up.
As torque is transmitted through the off-going friction element deceases, the automatic transmission output shaft torque drops, which causes the so-called torque hole. This is perceived by a vehicle occupant as an unpleasant shift shock. The inertia phase begins when the off-going clutch is released with no significant torque capacity.